In the cascaded coding and decoding of audio data, the spectral components of the short-time spectrum associated with a data block are formed within each codec stage of the cascade for each data block with a certain number of input data. A coded signal is then formed, by quantization and coding, on the basis of the spectral components for this data block and using a psycho-acoustic model to determine the bit distribution for the spectral components, whereupon time output data are obtained by decoding within the decoding part of the codec stage.
In the last few years, considerable advances have been made in the coding of sound signals with the least possible losses in sound quality. Such modern coding methods utilize the threshold of hearing of the human ear and try to adapt to the corresponding perceptual threshold the quantization noise generated by coding in such a way that, despite considerable data reduction, there is no audible deterioration. The coding and decoding devices which operate on this principle are also known as "perceptual codecs".
Such methods are suitable for a number of applications. They can be used to advantage practically everywhere where high-quality sound signals are to be stored or transmitted and the available capacity, e.g. the storage volume or the channel width, is to be used as effectively as possible.
Examples of such uses are the transmission of music over the ISDN telephone network, the storage of audio announcements or so-called "jingles" in flash ROM storage cards and the storage of music within music recorders using a so-called minidisk or the DCC method.
Examples of coding methods which use the above principle are those employed by the company Dolby Inc. under the names AC-2 and AC-3, the ATRAC method of the company Sony Corp. and the sound methods based on the standards ISO-MPEG (IS11172-3), layer 1-2-3.
All these processes are block-oriented, i.e. they analyse in each case a certain number of time input audio data, or audio sampling values, in other words a "data block", and determine for each block the spectral components present in the short-time spectrum associated with these data blocks. Afterwards the spectral components are quantized and coded, the coder employing a psycho-acoustic model to analyse the short-time spectrum so as to determine the bit distribution for the individual spectral components.
To summarize, one can also call this method "perceptual noise shaping": the noise induced by the quantization process is adapted to the perceptual threshold, the coder seeking to maintain a safe distance ("noise-to-mask ratio", NMR) from the estimated perceptual threshold.
In known methods for the coding and decoding of audio data, the sound quality of the audio signal on the output side deteriorates as the number of codec stages increases.
EP-A-0420745 contains a description of a coding device for generating digital audio signals in which the bandwidth for higher frequency ranges of the digital signals increases progressively and the coded signals for the various frequency ranges are formed in such a way that the number of sampled values within a block increases for higher frequency ranges. Quantization of the signals is achieved by assigning a certain number of bits to each of the bands.
The technical publication Alta Frequenza, Vol. XLVI, No. 8, August 1977, Milan, pp. 362-364 discusses the signal-noise ratio occurring in cascaded adaptive differential pulse code modulation codecs. The deterioration in the signal due to the multiple coding and decoding processes appears as a monotonically decreasing series.